Field effect transistors (FETs) are the basic building block of today's integrated circuit. Such transistors can be formed in conventional bulk substrates (such as silicon) or in semiconductor-on-insulator (SOI) substrates.
State of the art metal oxide semiconductor (MOS) transistors are fabricated by depositing a gate stack material over a gate dielectric and a substrate. Generally, the MOS transistor fabrication process implements lithography and etching processes to define the conductive, e.g., poly-Si, Si, gate structures. The gate structure and substrate are thermally oxidized, and, after this, source/drain extensions are formed by implantation. Sometimes the implant is performed using a spacer to create a specific distance between the gate and the implanted junction. In some instances, such as in the manufacture of an n-FET device, the source/drain extensions for the n-FET device are implanted with no spacer. For a p-FET device, the source/drain extensions are typically implanted with a spacer present. A thicker spacer is typically formed after the source/drain extensions have been implanted. The deep source/drain implants are then performed with the thick spacer present. High temperature anneals are performed to activate the junctions after which the source/drain and top portion of the gate are generally silicided. Silicide formation typically requires that a refractory metal be deposited on a Si-containing substrate followed by a high temperature thermal anneal process to produce the silicide material. The silicide process forms low resistivity contacts to the deep source/drain regions and the gate conductor.
In order to be able to make integrated circuits (ICs), such as memory, logic, and other devices, of higher integration density than currently feasible, one has to find a way to further downscale the dimensions of field effect transistors (FETs), such as metal oxide semiconductors. The downscaling of transistor dimensions allows for improved performance as well as compactness, but such downscaling has some device degrading effects. Generational improvements for high performance MOS devices are obtained by decreasing the transistor line width, reducing the gate oxide thickness, and decreasing the source/drain extension resistance. Smaller transistor line width results in less distance between the source and the drain. This results in faster switching speeds for complementary metal oxide semiconductor (CMOS) circuits. However, as the transistor line width gets smaller, the overall area available for silicidation is reduced. This means that as transistor line width shrinks, line resistance (i.e., series resistance) is increased. Increased line resistance causes degradation in device performance.
Source/drain extension resistance is another important performance factor. Drive currents may be increased by reducing source/drain extension resistance. Increasing the source/drain extension dose leads to lower resistance but has an undesirable side effect of increasing the junction depth.
As such, there is a need for providing a semiconductor structure having a low-resistance extension connection between the channel and the silicided source/drain regions with an independence from extension implants and device overlap (i.e., Miller) capacitance. Miller capacitance, which can also be referred to as the gate-drain or gate-source capacitance, increases the capacitance by a factor related to the voltage gain of a transistor.